/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.lucene.util;

import org.apache.lucene.search.DocIdSetIterator;

/** An iterator to iterate over set bits in an OpenBitSet.
 * This is faster than nextSetBit() for iterating over the complete set of bits,
 * especially when the density of the bits set is high.
 */
public class OpenBitSetIterator extends DocIdSetIterator {

  // The General Idea: instead of having an array per byte that has
  // the offsets of the next set bit, that array could be
  // packed inside a 32 bit integer (8 4 bit numbers).  That
  // should be faster than accessing an array for each index, and
  // the total array size is kept smaller (256*sizeof(int))=1K
  protected final static int[] bitlist={
    0x0, 0x1, 0x2, 0x21, 0x3, 0x31, 0x32, 0x321, 0x4, 0x41, 0x42, 0x421, 0x43, 
    0x431, 0x432, 0x4321, 0x5, 0x51, 0x52, 0x521, 0x53, 0x531, 0x532, 0x5321, 
    0x54, 0x541, 0x542, 0x5421, 0x543, 0x5431, 0x5432, 0x54321, 0x6, 0x61, 0x62, 
    0x621, 0x63, 0x631, 0x632, 0x6321, 0x64, 0x641, 0x642, 0x6421, 0x643, 
    0x6431, 0x6432, 0x64321, 0x65, 0x651, 0x652, 0x6521, 0x653, 0x6531, 0x6532, 
    0x65321, 0x654, 0x6541, 0x6542, 0x65421, 0x6543, 0x65431, 0x65432, 0x654321, 
    0x7, 0x71, 0x72, 0x721, 0x73, 0x731, 0x732, 0x7321, 0x74, 0x741, 0x742,
    0x7421, 0x743, 0x7431, 0x7432, 0x74321, 0x75, 0x751, 0x752, 0x7521, 0x753, 
    0x7531, 0x7532, 0x75321, 0x754, 0x7541, 0x7542, 0x75421, 0x7543, 0x75431, 
    0x75432, 0x754321, 0x76, 0x761, 0x762, 0x7621, 0x763, 0x7631, 0x7632, 
    0x76321, 0x764, 0x7641, 0x7642, 0x76421, 0x7643, 0x76431, 0x76432, 0x764321, 
    0x765, 0x7651, 0x7652, 0x76521, 0x7653, 0x76531, 0x76532, 0x765321, 0x7654, 
    0x76541, 0x76542, 0x765421, 0x76543, 0x765431, 0x765432, 0x7654321, 0x8, 
    0x81, 0x82, 0x821, 0x83, 0x831, 0x832, 0x8321, 0x84, 0x841, 0x842, 0x8421, 
    0x843, 0x8431, 0x8432, 0x84321, 0x85, 0x851, 0x852, 0x8521, 0x853, 0x8531, 
    0x8532, 0x85321, 0x854, 0x8541, 0x8542, 0x85421, 0x8543, 0x85431, 0x85432, 
    0x854321, 0x86, 0x861, 0x862, 0x8621, 0x863, 0x8631, 0x8632, 0x86321, 0x864, 
    0x8641, 0x8642, 0x86421, 0x8643, 0x86431, 0x86432, 0x864321, 0x865, 0x8651, 
    0x8652, 0x86521, 0x8653, 0x86531, 0x86532, 0x865321, 0x8654, 0x86541, 
    0x86542, 0x865421, 0x86543, 0x865431, 0x865432, 0x8654321, 0x87, 0x871, 
    0x872, 0x8721, 0x873, 0x8731, 0x8732, 0x87321, 0x874, 0x8741, 0x8742, 
    0x87421, 0x8743, 0x87431, 0x87432, 0x874321, 0x875, 0x8751, 0x8752, 0x87521, 
    0x8753, 0x87531, 0x87532, 0x875321, 0x8754, 0x87541, 0x87542, 0x875421, 
    0x87543, 0x875431, 0x875432, 0x8754321, 0x876, 0x8761, 0x8762, 0x87621, 
    0x8763, 0x87631, 0x87632, 0x876321, 0x8764, 0x87641, 0x87642, 0x876421, 
    0x87643, 0x876431, 0x876432, 0x8764321, 0x8765, 0x87651, 0x87652, 0x876521, 
    0x87653, 0x876531, 0x876532, 0x8765321, 0x87654, 0x876541, 0x876542, 
    0x8765421, 0x876543, 0x8765431, 0x8765432, 0x87654321
  };
  /***** the python code that generated bitlist
  def bits2int(val):
  arr=0
  for shift in range(8,0,-1):
    if val & 0x80:
      arr = (arr << 4) | shift
    val = val << 1
  return arr

  def int_table():
    tbl = [ hex(bits2int(val)).strip('L') for val in range(256) ]
    return ','.join(tbl)
  ******/

  // hmmm, what about an iterator that finds zeros though,
  // or a reverse iterator... should they be separate classes
  // for efficiency, or have a common root interface?  (or
  // maybe both?  could ask for a SetBitsIterator, etc...

  final long[] arr;
  final int words;
  private int i=-1;
  private long word;
  private int wordShift;
  private int indexArray;
  private int curDocId = -1;

  public OpenBitSetIterator(OpenBitSet obs) {
    this(obs.getBits(), obs.getNumWords());
  }

  public OpenBitSetIterator(long[] bits, int numWords) {
    arr = bits;
    words = numWords;
  }

  // 64 bit shifts
  private void shift() {
    if ((int)word ==0) {wordShift +=32; word = word >>>32; }
    if ((word & 0x0000FFFF) == 0) { wordShift +=16; word >>>=16; }
    if ((word & 0x000000FF) == 0) { wordShift +=8; word >>>=8; }
    indexArray = bitlist[(int)word & 0xff];
  }

  /***** alternate shift implementations
  // 32 bit shifts, but a long shift needed at the end
  private void shift2() {
    int y = (int)word;
    if (y==0) {wordShift +=32; y = (int)(word >>>32); }
    if ((y & 0x0000FFFF) == 0) { wordShift +=16; y>>>=16; }
    if ((y & 0x000000FF) == 0) { wordShift +=8; y>>>=8; }
    indexArray = bitlist[y & 0xff];
    word >>>= (wordShift +1);
  }

  private void shift3() {
    int lower = (int)word;
    int lowByte = lower & 0xff;
    if (lowByte != 0) {
      indexArray=bitlist[lowByte];
      return;
    }
    shift();
  }
  ******/

  @Override
  public int nextDoc() {
    if (indexArray == 0) {
      if (word != 0) {
        word >>>= 8;
        wordShift += 8;
      }

      while (word == 0) {
        if (++i >= words) {
          return curDocId = NO_MORE_DOCS;
        }
        word = arr[i];
        wordShift = -1; // loop invariant code motion should move this
      }

      // after the first time, should I go with a linear search, or
      // stick with the binary search in shift?
      shift();
    }

    int bitIndex = (indexArray & 0x0f) + wordShift;
    indexArray >>>= 4;
    // should i<<6 be cached as a separate variable?
    // it would only save one cycle in the best circumstances.
    return curDocId = (i<<6) + bitIndex;
  }
  
  @Override
  public int advance(int target) {
    indexArray = 0;
    i = target >> 6;
    if (i >= words) {
      word = 0; // setup so next() will also return -1
      return curDocId = NO_MORE_DOCS;
    }
    wordShift = target & 0x3f;
    word = arr[i] >>> wordShift;
    if (word != 0) {
      wordShift--; // compensate for 1 based arrIndex
    } else {
      while (word == 0) {
        if (++i >= words) {
          return curDocId = NO_MORE_DOCS;
        }
        word = arr[i];
      }
      wordShift = -1;
    }

    shift();

    int bitIndex = (indexArray & 0x0f) + wordShift;
    indexArray >>>= 4;
    // should i<<6 be cached as a separate variable?
    // it would only save one cycle in the best circumstances.
    return curDocId = (i<<6) + bitIndex;
  }

  @Override
  public int docID() {
    return curDocId;
  }
  
}
